1. Field of the Invention
The present invention relates to a storage-volume fuel injection system for an internal combustion engine.
2. Technical Background
Fuel injection systems of modern internal combustion engines normally comprise a pump for supplying high-pressure fuel to a common rail having a given fuel storage volume and for supplying a number of injectors associated with the engine cylinders. The pump comprises at least one reciprocating pumping member, which, each time, performs an intake stroke and a compression or delivery stroke.
As is known, to be atomized properly, the fuel must be brought to extremely high pressure, e.g. of about 1600 bars in maximum-load conditions. Recent standards governing the pollutant content of engine exhaust gas require that the fuel feed pressure to the injectors be reproducible as accurately as possible, with respect to the electronic central control unit map, in “cylinder-cylinder” mode and “injection-injection” mode into the same cylinder, particularly in low/medium load conditions, which are the most important in evaluating the pollutant content of exhaust gas. Fluctuations with respect to the set pressure in the common rail can be limited, if the volume of the common rail is over one hundred times the quantity of fuel drawn by each injector at each combustion cycle. Such a common rail, however, is normally extremely bulky and therefore difficult to install on the engine.
To control the pressure in the common rail as mapped in the central control unit, injection systems have been proposed comprising a bypass solenoid valve fitted to the delivery line between the pump and the common rail, and which is controlled by an electronic unit on the basis of various engine operating parameters. Another proposal is to operate the pumping member by means of a cam synchronized with operation of each injector.
These known systems have the drawback of keeping the pump operating permanently at maximum flow, since the bypass solenoid valve provides simply for draining into the tank the fuel pumped in excess of that drawn by the injectors, thus dissipating thermal energy.
As is known, when the engine is started, the fuel, before it is ignited, must be brought to a minimum pressure sufficient to ensure reasonable atomization of the fuel injected at this stage. The size of the storage volume, however, does not allow an acceptable reduction in start-up time, so that fuel is injected at less than optimum pressure, thus resulting in poor combustion at the initial engine operating stage.
This practice has the drawback of failing to conform with recent pollution standards, which, in evaluating the pollutant content of exhaust gas, also take into account those emitted at the initial engine operating stage. To conform with current standards, it is therefore essential that the fuel also be brought to the right pressure at the initial operating stage of the engine.
Finally, a high-pressure pump is known featuring two side by side pumping members, each having an intake and compression chamber, and in which a bypass valve is located between the two chambers of the two cylinders, and is operated to drain into the chamber of one cylinder the surplus fuel pumped by the other cylinder.
This pump has the drawback of communication between the two compression chambers, one at high and the other at low pressure, possibly resulting in cavitation, thus reducing fill of the compression chamber and so impairing volumetric efficiency. To limit the drawback, additional pre-feed pumps with very high operating pressures, of around 20 bars, must be used, thus posing problems in terms of the working life of the pre-feed pump.